(1) Field of the Invention
This invention pertains to the art of fluid conditioning apparatus, systems and methods, and is more particularly concerned with air cycle refrigeration apparatus employed in air conditioning systems for the ventilation air distribution to the cabins or other compartments of aircraft.
(2) Description of the Prior Art
Life support air for the occupants of an aircraft cabin or compartment is extracted from the ambient atmosphere, after which it is compressed by the engine or engines of the aircraft or by an auxiliary power unit or units, cooled by heat exchange with ambient ram air, cooled further by adiabatic expansion in an expansion turbine, tempered by addition of warm bypass air if necessary, and then admitted to the cabin air induction and distribution system. This basic step-by-step treatment of the air is inherent in practically all of the more complex systems ranging from various arrangements of simple systems through the more elaborate bootstrap and shoe-string systems, and the like. The air extracted from ambient usually has entrained moisture of varying amounts (and which may be present in vapor, mist or droplet form) which must be accounted for in the design of the air conditioning system, primarily because the entrained moisture condenses and usually freezes at some point downstream of the expansion thereof.
Furthermore, the efficiency of the system is greatly enhanced where the system design results in turbine air outlet temperatures as near to, or even below, the freezing point of water.
As much as possible of the entrained moisture in the air is removed in the air conditioning system by means of coalescers, water separators, water traps and the like. One hundred percent removal is not possible, hence the introduction of the air from the system into the cabin air distribution ducts, according to the prior art, takes place with the air at a temperature above freezing point of water in order to minimize icing in the downstream air ducts. As stated, warm bypass air is modulated into the airstream to provide low limiting of the duct air to a temperature of about 35.degree.-37.degree. F.
Systems of the prior art employing the basic step-by-step method briefly outlined above, are subject to certain limiting factors which result in degradation of system performance and lowered efficiency. For example, it is known that the cooling capacity of a particular system can be greatly enhanced by operating the expansion turbine with exhaust air temperatures below the freezing point of water. Yet to do so would require greatly increased hot bypass air and thus result in serious degradation of performance. If, on the other hand, the amount of hot bypass air (required to temper the turbine exhaust air) is to be held at a bare minimum, then the expanded air has to be at a temperature above freezing of the entrained water vapor, mist, or droplets. This means that the turbine and associated components of the air conditioning unit are not operating in an efficient manner under conditions which may be dictated by a broad range of parameters of a problem statement.